Characterization of hydrogen absorption/desorption states on lithium-carbon-hydrogen system by neutron diffraction

The nanostructural hydrogenated graphite (C^nano H_x) was synthesized from graphite by ball milling under hydrogen (H₂) atmosphere. In this product, characteristic hydrogenated states in the form of polarized hydrocarbon groups (_CH,_C H₂, and_C H3) are realized in the nanoscale. By synthesizing the composite of C^nano Hx and lithium hydride (LiH), known as the Li_C_H system, hydrogen was desorbed at 350 °C, which is a lower temperature compared to the decomposition temperature of each component. It is considered that this hydrogen desorption would be induced by destabilization of each hydrogen absorbed state due to an interaction between the polarized C_H groups in C^nano Hx and LiH. Therefore, in order to understand the hydrogen absorption/desorption mechanism of the Li_C_H system, it is an important issue to investigate the change in the C_H groups during hydrogen absorption/desorption reactions in the composite. The correlations among atoms contained in this composite are examined by neutron diffraction measurements, where the protium/deuterium (H/D) isotopic substitution was used to clarify the location of hydrogen atoms in this composite. Some C_D and Li_D correlations are found from the radial distribution function [RDF (r)] obtained by the neutron diffraction for the C^nano Dx and LiD composite. After dehydrogenation, C_C triple bond and Li_C bond, ascribed to lithium carbide (Li2 C2), are observed. Furthermore, the RDF (r) corresponding to rehydrogenated composite indicates the presence of not only the Li_D correlation but also the C_D one.